Piezoelectric transducers are widely used to convert between electrical and acoustical signals. If a single voltage is applied across opposite sides of a thin disk of piezoelectric material, the dimensions of the disk will vary with the voltage thereby generating an acoustic signal. Conversely if acoustic waves impinge upon the disk, it will be mechanically deformed and a voltage will appear across the two sides.
A problem which has plagued the use of piezoelectric transducers for many applications is that the efficiency of transduction between electrical and acoustical signals depends strongly on the frequency of the exciting signal, be it electric or acoustic. This effect is particularly pronounced when the exciting frequency approaches a mechanical resonance frequency of the transducer. For applications involving short ultrasonic pulses such as acoustic imaging and sonar the problem of the narrow bandwidth of piezoelectric transducers is particularly troublesome since for an electroacoustic converter to avoid distorting pulses it must have a constant conversion efficiency and linear phase transfer relation over a relatively wide frequency range.
Several techniques have been employed to permit the use of thin-disk piezoelectric transducers in applications requiring broadband electroacoustic converters. One technique involves attaching a sound-absorbing backing on the transducer, as disclosed in Kossoff, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-13, pp. 20-30(March, 1966) and in Merkulov and Yablonik, Soviet Physics-Acoustics, Vol. 9, pp. 365-372(April-June, 1964). While damping the transducer with a sound-absorbant backing does broaden its bandwidth, the conversion efficiency of the transducer is greatly reduced since the backing must absorb a large proportion of the acoustic signal. This is a serious drawback in many applications, particularly those involving the detection of weak acoustic signals. Furthermore the backing increases the physical size of the transducer making it too bulky for some applications.
A second technique for increasing the bandwidth involves inserting a layer of material between the transducer and the acoustic medium with which the transducer is to communicate, as disclosed in the Kossoff reference cited above. The thickness and acoustic impedance of the material are selected to transform the acoustic impedance of the transducer material to that of the medium. This impedance matching may be accomplished at a selected frequency if the acoustic impedance of the matching layer equals the square root of the product of the impedances of the transducer and the load medium and if the thickness of the matching layer equals the acoustic quarter wavelength at the selected frequency in the material of which the matching layer is composed. Although such matching layers increase the bandwidth of the transducer to a limited extent, materials which have the proper acoustic impedance are not readily available to impedance match some important classes of transducers and load media. One widely-used group of piezoelectric transducers have acoustic impedances in the range of from about 30 .times. 10.sup.6 to about 36 .times. 10.sup.6 kg/s m.sup.2. Examples of such materials are lithium niobate and the lead zirconate-titanate ceramics currently marketed by the Vernitron Corporation of 232 Forbes Road, Bedford, Ohio, under the trade names "PZT-4," "PZT-5A," "PZT-5H" and "PZT-7A." Physical constants characterizing these four "PZT" ceramics are set forth in Table I on page 21 of the article by Kossoff cited above, which table is incorporated herein by reference for purposes of identifying the "PZT" ceramics. If a transducer of one of these materials is to be impedance matched with a single quarter-wave layer to a water medium, which has an acoustic impedance of about 1.5 .times. 10.sup.6 kg/s m.sup.2, a material having an acoustic impedance of roughly 7 .times. 10.sup.6 kg/s m.sup.2 is required. Materials of this impedance, however, are not readily available and must be specially synthesized for this application. Furthermore the bandwidth achieved with an electroacoustic converter made from a piezoelectric transducer and a single quarter-wave matching layer, although greater than the bandwidth of the transducer along radiating into a water load, may not be broad enough for many applications involving short acoustic pulses.
Electroacoustic converters employing double-layer acoustic couplers have been reported, but these devices failed to exhibit sufficiently broad bandwidth or low insertion loss for many applications. One reference; Dianov, Soviet Physics-Acoustics, Vol. 5, pp. 30-35(1959); discloses the insertion of a layer of water and a glass plate between a quartz transducer and a water load, the thickness of the glass plate being half the wave thickness of the quartz transducer and the thickness of the water layer being variable. The author noted that the presence of the layers led "to a marked reduction in the transmission band." U.S. Pat. No. 2,430,013 discloses an electroacoustic converter for use with water loads which employs a quartz transducer and a broadband double-layer acoustic coupler. As discussed below, because of the high radiation Q of acoustically-matched quartz transducers, the insertion loss of broadband quartz converters is generally too high for many applications. In the Kossoff article cited above, it is stated that, "it was experimentally confirmed that the response [of a piezoelectric ceramic transducer] was degraded when a double matching layer consisting of an outer .lambda./4 [quarter-wave] araldite layer on the .lambda./4 matching aluminum araldite layer was employed." None of these references in any way discloses or suggests the novel electroacoustic converter as disclosed and claimed herein.